 Migmatites or diatextites, are very high-grade metamorphic rocks that result from high temperature/high pressure prograde Barrovian regional metamorphism.
 Migmatites comprise both leucosomes, which contain new minerals recrystallized due to the incipient melting, together with restite mesosomes, which contain refractory mafic minerals that resisted in situ melting. Younger mineral bodies comprising fusible recrystallized minerals are also referred to as neosomes, while older bodies of refractory minerals are paleosomes. These are distinguished from xenoliths, which are fragments of metamorphically altered country rock that fell into magma or lava and become enveloped within igneous rock. In contrast to light leucosomes, darker mesosomes are intermediate in color, and the darkest bodies are termed melanosomes.
Typical restite minerals include refractory amphiboles, biotite, pyroxenes, plagioclase feldspars, and ilmenite or other iron oxides that fail to melt when more fusible leucosomal minerals melt at the high Barrovian temperatures and pressures that generate migmatites. High pressures have warped the veins and layers of minerals in migmatites, and the darker rock has been intruded by lighter colored, leucosomal veins of quartz and feldspars. Migmatites often appear as tight dikelets, veins, and segregations of light colored leucosomal granitic minerals, within dark colored amphibole and biotite minerals of the mesosome (comp. melanosome). Migmatites have gneissic composition, and those migmatites that have suffered extreme near melting conditions display anatexic loss of texture as they merge toward granite. Restites in S-type granites results from intracrustal melting of a typical metamorphic mineral assemblage of high-pressure gneiss of sedimentary origin. 
Because migmatites result from near melting of protoliths, they have not crystallized from totally molten material, and so do result from solid-state reactions. Commonly, migmatites occur within extremely deformed rocks that formed at the base of eroded, orogenic belts, typically within Precambrian cratonic blocks. Migmatite textures result from this thermal softening: schlieren textures are common within granite-associated migmatites, and are often seen in restite xenoliths and around the margins of S-type granites. ptygmatic folds (image above right) result from highly plastic ductile deformation of the gneissic banding, so they bear little or no relationship to the defined foliation of most regular folds. Ptygmatic folds can remain restricted to compositional zones of the migmatite, such as seen in fine-grained shale protoliths, rather than in coarse granoblastic sandy protolith. |
links: images: hand-specimens: migmatite, 2, augen migmatite (cn); formations: migmatite boulder, Saaremaa; boulder with ptygmatic folds, 2; contact metamorphism adjacent to granodiorite plug in the Dezadeash Range with fragments of KMA cordierite gneiss surrounded by granodioritic melt, leucosomes within the gneiss (red arrow) are indicators that the gneiss itself is starting to melt at the beginning of migmatization, 2; 3Ga migmatite gneiss cut by 2.7 Ga granite, exposed in outcrop near Michigamme, Michigan; granitic migmatite, Duoxiong La Pass near Namche Barwa; granite formed from crystallised melt, darker rocks are unmelted restites of the original metamorphosed sediment, Cape Town, South Africa; metamorphic melts, migmatite, 2, 3, 4, (boudin, 2, 3, 4, 5); migmatite (close), Série métapélitique de l'Agly; layered gneisses and amphibolites, Liberdade Nappe; Mount Arabia Migmatite, Lithonia Gneiss, Arabia Mountain, southeastern DeKalb County; Tertiary leucocratic granodioritic migmatite, neosome is leucocratic granodiorite; migmatitic pelitic gneiss of the Mt. Cuba Wissahickon formation, typically pelitic gneiss composed of biotite, garnet, sillimanite, plagioclase and orthoclase feldspar, and quartz, and representing a high temperature and low pressure assemblage of minerals in region considered part of a forarc accretionary complex known as "Arc Wissahickon"; migmatite N. Dabie Zone; migmatite, Cape Peninsula; high-grade banded gneisses and migmatites in Filchnerfjella, Antarctica; close-up: undeformed garnet + K-feldspar migmatite leucosomes in Namaqualand granulite; compositional layering in metapelitic gneiss cut by narrow planar migmatite leucosomes; Migmatite, Wind Rivers Wyoming; migmatitic biotite gneiss in Namaqualand - of semi-pelitic composition, light-coloured neosome consists of streaks and patches of coarse quartz and feldspar with large garnets (no melanosome), the unfoliated neosomes have indistinct boundaries and although they approximately follow the foliation in the rock they are locally discordant; Liberdade Nappe - stromatic migmatites contain large garnets with rutile inclusions, abundant kyanite, K-feldspar, quartz, plagioclase, biotite and scarce muscovite; Mount Arabia Migmatite, Lithonia Gneiss, Arabia Mountain, in southeastern DeKalb County; Archean migmatites, outcrop near Morton, Minnesota with light colored granitic layers (coarse feldspar and quartz) and dark colored amphibolites and more mafic gneiss; vein; buckled migmatite; migmatite; biotite gneisses are varyingly migmatized to have a "igneous appearing" texture and structure; jp; ?calcsilicate migmatite?; migmatite in Kwinitsa gneiss, B.C; migmatite in Antarctica, and melt patches contain biotite aggregates that may be pseudomorphs after garnet; thin-sections: LPA view of the black solid residue portion of migmatite, showing amphiboles and altered feldspars, and LPNA view; webpages: Granulites and Migmatites; image library, geological images, all (Jaeger, Uni Melb.); geophysics image gallery; see M section |
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